Photochemical formation of nitrated and oxygenated polycyclic aromatic compounds over the Southern Ocean

Polycyclic aromatic hydrocarbons (PACs) can be secondarily transformed through photochemical reactions into nitrated and oxygenated derivatives (N-/O-PACs), which are often more toxic than their parent compounds. However, the conversion mechanism over marine boundary layer (MBL) is still unclear. Here, air samples were collected in MBL of the climate-sensitive regions (Southern Ocean, SO), and N-/O-PACs were determined in both the gas phase and total suspended particles (TSP). The results showed stronger photochemical formation trends of N-/O-PACs in TSP than in the gas phase, highlighting the dominant role of heterogeneous processes in photochemical formation. In the gas phase, O-PACs exhibited higher formation trends than N-PACs, whereas N-PACs more often exceeded O-PACs in TSP. A generalized additive model was further applied to examine their photochemical formation trends and identify the major environmental controls. Wet deposition capacity of atmospheric was identified as an important factor affecting the gas phase photochemical dependence. In contrast, OH radical intensity and radiation flux played dominant roles in the N-/O-PACs formation in TSP. The model results further indicated that the formation of N-/O-PACs was only weakly correlated with NO₂, suggesting that NO₂-initiated atmospheric oxidation was relatively weak in SO, whereas other oxidation pathways were dominant.